There is nothing inherently harmful about doing two workout sessions in a row. It is harmful if it is more work than you can do, or more work you can recover from in time for the next time you do a workout.

Just listen to your body, really. If you feel too messed up after working out to train more, don't. If you feel much worse after doing the extra work, don't do it again.

Otherwise, yeah, go for it. I train kickboxing after I do my workout...it's only a problem if my coach has us all do squats, pushups, pullups, and v-ups for 3 minute rounds, because that heavily overlaps the workout I do before class. If not, I can go with no problems.

Just take it slowly and pay attention to your own signals of exhaustion or adaptation and go for it!